Method of electroplating bismuth on steel and electrolyte therefor



United States Patent 3,256,160 METHOD OF ELECTROPLATING BISMUTH 0N STEEL AND ELECTROLYTE THEREFOR Jack A. McCarthy, Monroeville, Pa., assignor to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Sept. 4, 1962, Ser. No. 221,340 9 Claims. (Cl. 204-45) This invention relates to the electrodeposition of bismuth and more'particularly to improved plating baths for this purpose which are adapted to deposit tightly adherent coatings of bismuth directly on ferrous base metals.

Although bismuth possesses unusual properties including exceptionally high corrosion resistance, its use as a coating for iron and steel has been limited by difficulties arising in electrodeposition of the metal. The most successful bismuth plating baths known heretofore have been acidic solutions of the metal. These provide acceptable deposits on copper or nickel but when used to plate directly on a ferrous base metal, produces a powdery immersion-deposit which precludes adhesion of the subsequently electrodeposited coating. Alkaline baths, prepared by dissolving bismuth salts in a sodium or potassium hydroxide solution, are too viscous and have proven operable only at extremely low current densities. Cyanide based electrolytes also fail to dissolve sufficient bismuth to afford a practical plating solution; while tr-iethanolamine baths, quickly form an insulating film on the bismuth anodes which interferes with current flow to further complicate the problem of maintaining a practical concentration of bismuth in solution. The alkaline tartrate solutions produce nonadherent powdery deposits.

The aforementioned deficiencies are absent in the plating baths of the present invention. The need for undercoats of copper or nickel when plating ferrous base metals is eliminated. The new baths'are easy to maintain and produce lustrous, tightly adherent deposits over a wide tange of current densities, 5-50 amps per square foot, and bath temperatures, 40 to 200 F.

Essentially, the electrolytes of my invention are aqueous alkaline solutions of bismuth, in amount between about 6 to 85 g./l. when calculated as metallic bismuth, and a chelating agent selected from the group comprising gluconic acid, the alkali-metal pyrophosphates, the alkalimetal ethylenediamine-tetraacetates and mixtures of the last two named salts. Baths compounded from the pyrophosphates, the ethylenediamine-tetraacetates and mixtures. thereof preferably contain, in addition, a small amount of dextrin, preferably about 10 grams per liter of bath, which acts to improve the smoothness, adhesion and luster of the deposits.

For the best results, the pH of the baths should be maintained above 9, preferably in the range 9-11, and this becomes a critical requirement when plating directly on steel or other ferrous base metals. Moreover, ,although usable deposits can be produced on base metals other than ferrous basemetals at values of pH down to about 7, the quality of the plate, particularly its smoothness, deteriorates as the alkalinity is reduced below pH 9. The alkalinity is adjusted by additions of an alkali-metal hydroxide and hydrochloric acid as required; the amount of base or acid added depends on the choice of chelating agent and its concentration in the bath.

In solutions of the proper alkalinity, chelating agents of the group set forth above act to sequester the bismuth ion without adversely affecting the solubility of the metal, thus solutions containing an adequate concentration of bismuth for practical plating operations are provided while the tendency toward uncontrolled immersiondeposition of the metal is effectively suppressed. A concentration of at least 100 g./l. of chelating agent is reice quired; while the amounts can be increased up to the solubility limit no practical benefits are derived at concentrations above the maximums recited in the following examples:

Example I G./l. Potassium pyrophosphate -300 Bismuth chloride (BiCl -H O) 10-60 Sodium hydroxide or hydrochloric acid, suflicient to adjust pH to 9-11. Dextrin 1-50 Example II Tetrasodium ethylenediamine-tetraacetate 100-250 Bismuth chloride (BiCI -H O) 10-100 Sodium hydroxide of hydrochloric acid, sufficient to adjust pH to 9-11. Dextrin 1-50 Example III 50% gluconic acid 100-300 Bismuth chloride (BiCl -H O) 15-80 Sodium hydroxide, suflicient to raise pH to 9-11.

Example IV Potassium pyrophosphate 100-300 Tetrasodium ethylenediamine-tetraacetate 50-250 Bismuth chloride (BiCl -H O), 10-140 Hydrochloric acid, sufficient to lower pH to about 10.0. Dextrin l-50 Example V i Q Potassium pyrophosphate 100-300 Disodium ethylenediamine-tetraacetate 50-250 Bismuth chloride (BiCl -I-I O) 15-140 Sodium hydroxide, sufiicient to raise pH to 9-10. Dextrin 1-50 The baths may be prepared by adding the chelating agent and bismuth chloride to water heated to at least F., perfectly boiling. The temperature should be maintained for about 1 hour. When the solution clears, the necessary. acid, concentrated HCl or hydroxide, sodium hydroxide (200 g.l. strength) may be added. The solution should be stirred constantly during this addition and the stirring continued until solution is again substantiallyclear. If need be, the solution should then be filtered to remove any remaining precipitate; afterwhich the dextrin, previously dissolved in boiling water, may be added. While the chloride salt is a convenient and the preferred source of bismuth, bismuth pyrophosphate or any other water soluble compound of bismuth, the acid radical of which does not cause interfering reaction at the electrodes, may be substituted.

Plating is accomplished by connecting the clean base metal object as the cathode in any one of the above solutions. The anodes may be either bismuth or carbon. If bismuth anodes are used, anode efliciency can be increased by the addition of a small amount of sodium chloride to the bath.

The deposits from. the pyrophosphate baths of Example I have a pinkish cast; those from the gluconic baths of Example III and the ethylenediamine-tetraacetate solutions of Example II, a brownish tone. Baths containing both pyrophosphate and ethylenediamine-tetraacetate, Examples IV and V, produce deposits of a lighter brownish cast. As removed from the bath, all of the coatings carry a light smut. This is readily removed by rinsing and lightly wiping the surface of the plate; the underlying coating is glossy and tightly adherent. The smut is essentially pure bismuth. Although it does not adversely affect the quality of the plating, it represents a loss of metallics and the quantity thereof increases with the current density and bath temperature. Accordingly, while all of the baths produce satisfactory coatings at temperatures up to 200 F. and current densities up to at least 50 amps per square foot, it is preferable to conduct the plating at room temperature and to use an intermediate current density of about 25 amps per square foot.

Of the various bath compositions encompassed in the foregoing examples, I prefer when practicing my invention and particularly when plating directly on steel or other ferrous alloy, the following composition:

G./l. Potassium pyrophosphate 200 Tetrasodium ethylenediamine-tetraacetate 150 Bismuth chloride (BiCl -I-l O) 48 Hydrochloric acid, suflicient to reduce pH to 10.

Dextrin This bath, when operated at room temperature and a current density of to amps per square foot, produces tightly adherent coatings of exceptional gloss, good color and a minimum of smut. As in all coating operations best results require the base metal to be scrupulously clean prior to plating; a condition which is readily achieved by any of several well known cleaning practices, the choice of which depends largely on the type of soil on the surfaces of the base metal, and the preference of the individual plater.

While I have described certain specific embodiments of my invention, it is apparent that modifications may arise. Therefore, I do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

I claim:

1. A plating bath for the electrodeposition of bismuth consisting essentially of an aqueous solution having a pH between 7 and 11 containing bismuth and an effective amount of chelating salt from the group oonsisitng of alkali-metal pyrophosphates, alkali-metal ethylenediamine-tetraacetates and mixtures thereof.

2. A plating bath for the electrodeposition of bismuth consisting essentially of an aqueous solution having a pH between 7 and 11 containing 6 to 85 grams bismuth per litre and at least 100 grams per litre of a chelating salt from the group consisting of alkali-metal pyrophosphates, alkali-metal ethylenediamine-tetraacetates and mixtures thereof.

3. A plating bath for the electrodeposition of bismuth consisting of an aqueous solution of:

Potassium pyrophosphate 100-300 Bismuth chloride (BiCl -H O) 10-60 Dextrin 1-50 adjusted to a pH in the range 9 to 11.

4. A plating bath for the electrodeposition of bismuth consisting of an aqueous solution of:

G./l. Tetrasodium ethylenediamine-tetraacetate 100-250 Bismuth chloride (BiCl -H O) 10-100 Dextrin l-SO adiusted to a pH in the range 9 to 11.

4 5. A plating bath for the electrodeposition of bismuth consisting of an aqueous solution of:

6. A plating bath for the electrodeposition of bismuth consisting of an aqueous solution of:

Potassium pyrophosphate 100-300 Disodium ethylenediamine-tetraacetate 50-250 Bismuth chloride (BiCl II O) 15-140 Sodium hydroxide, sufiicient to raise pH to 9-10.

Dextrin 1-50 7. A method for electrodepositing bismuth comprising disposing an article adapted to receive an electrodeposited coating as a cathode in an aqueous bath of electrolyte consisting essentially of'6 to grams of bismuth per litre of electrolyte and at least grams per line of a ch elating salt from the group consisting of alkali-metal pyrophosphates, alkali-metal ethylenediamine-tetraacetates and mixtures thereof, maintaining the pH of said solution between 7 and 11 and its temperature between 40 and 200 F., applying electric current to said bath and maintaining the current density between 5 and 50 amps. per square foot of surface area of said article.

8. A method of electrodepositing bismuth on ferrous articles comprising disposing the ferrous article as a cathode in an aqueous bath of electrolyte consisting essentially of 6 to 85 grams of bismuth per litre of electrolyte and at least 100 grams per litre of a chelating salt from the group consisting of alkali-metal pyrophosphates, alkali-metal ethylenediamine-tetracetates and mix-tures thereof, maintaining the pH of said solution between 9 and 11 and its temperature between 40 and 200 F., applying electric current to said bath and maintaining the current density between 5 and 50 amps per square foot of surface area of said article.

9. A method of electrodepositing bismuth on ferrous articles comprising disposing the ferrous article as a cathode in an aqueous bath of electrolyte consisting essentially of G./l. Potassium pyrophosphate 200 Tetrasodium ethylenediamine-tetraacetate Bismuth chloride (BiCl -H O) 48 Hydrochloric acid, sufiicient to reduce pH to 10. Dextrin 10 maintaining said electrolyte at room temperature, applying electric current to said bath and maintaining the current density between 5 and 25 amps per square foot of surface area of said article.

References Cited by the Examiner UNITED STATES PATENTS 2,801,959 8/1957 Du Rose 204-45 FOREIGN PATENTS 4,947 11/1881 Great Britain.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. A PLATING BATH FOR THE ELECTRODEPOSITION OF BISMUTH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION HAVING A PH BETWEEN 7 AND 11 CONTAINING BISMUTH AND AN EFFECTIVE AMOUNT OF CHELATING SALT FROM THE GROUP CONSISTING OF ALKALI-METAL PYROPHOSPHATES, ALKALI-METAL ETHYLENEDIAMINE-TETRAACETATES AND MIXTURES THEREOF. 